A source material commonly used for the electrolytic production of magnesium metal is a spray-dried magnesium chloride salt derived from sea water, inland salt lakes, salt wells, and the like. This spray-dried magnesium salt will comprise magnesium chloride, as a major portion, plus magnesium chloride complexes containing one or more hydroxyl groups, significant amounts of particulate MgO and trace amounts of other oxides and/or metals such as sodium, potassium, calcium, chromium, manganese and nickel; and while the presence of the latter metals in the spray-dried magnesium salt has no significant deleterious effect on the quality of the magnesium metal produced electrolytically from the fused salt, it is essential that any oxygen containing compounds, hydroxyl groups and the like be substantially completely removed-for otherwise they will interfere with the reactions at the electrodes at temperatures prevailing in the cell thereby reducing current efficiencies, shortening cell life and causing difficulties attending recovery of the magnesium metal from the cell.
Several methods have been employed heretofore for purifying spray-dried magnesium chloride using chlorine to chlorinate and/or dissipate any oxygen containing compounds including hydroxyl groups so that the purified salt may be used as an electrolyte inthe electrolytic production of magnesium metal.
U. S. Pat. No. 3,484,191, describes a process for purifying hygroscopic magnesium chloride salts derived from ocean water, inland seas, salt lakes, and similar sources, by treating the impure magnesium chloride, in a fused state, with a sulfur chloride compound such as sulfur monochloride, sulfur dichloride, or sulfur tetrachloride, wherein the sulfur combines with any oxygen containing compounds present in the fused magnesium chloride to form sulfur dioxide gas while the liberated chlorine reacts with any magnesium oxide present to form magnesium chloride.
U. S. Pat. No. 3,512,928, is similarly directed to a process for purifying hygroscopic magnesium chloride salts so as to eliminate soluble and reactibe oxygen containing compounds therefrom, wherein activated carbon, or carbon bearing materials which produce activated carbon are introduced into a bath of the fused magnesium chloride and react, selectively, with any sulfates or soluble and reactive oxygen bearing compounds in the fused salt bath to dissipate said compounds in the form of gaseous CO, H.sub.2 O, CO.sub.2, CO.sub.2, SO.sub.2, and the like.
Although the methods disclosed in the prior art have had some measure of success, the economics of producing electrolytic magnesium metal, using a spray-dried magnesium chloride source material, are extremely demanding and hence, efforts are constantly being made to achieve more efficient and economical methods for purifying a spray-dried magnesium chloride source material.